1. Field of the Invention
The present invention relates to a rotating electrical machine having a stator core formed of steel plates that are deformed by bending into a cylindrical shape with their opposite end faces being fusion bonded to each other.
2. Description of the Related Art
In the past, as a rotating electrical machine in the form of a vehicular alternator, there has been known one which includes a rotor having N poles and S poles alternately formed along a circumferential direction thereof, a stator core arranged so as to surround the rotor and having a plurality of axially extending slots formed at a predetermined pitch in the circumferential direction, and a stator having a stator winding fitted in the slots, wherein the stator core is formed by first deforming through bending into a cylindrical shape an element iron core of a hexahedral shape, which is formed of a predetermined number of laminated rolled steel plates with their outer peripheries joined together by laser welding (lamination welding), and then fusion binding the opposite ends of the element iron core to each other while placing them into abutment with each other (see, for example, a first patent document: Japanese patent application laid-open No. 2001-245446 (FIG. 26 and FIG. 40)).
In this case, the attachment of the stator winding to the stator core is performed by inserting individual straight portions of groups of belt-shaped element wires, which together constitute the stator winding, into the individual slots, respectively, in the element iron core, so there are the following advantages. That is, the efficiency or easiness of the attachment is high, the damage of the stator winding is small, and the space factor of conductors of the stator winding in the slots is high.
In addition, the above-mentioned element iron core is divided into parts along a diametral or radial direction in a tooth formed between adjacent slots, respectively, and the dividing surfaces thereof are placed into abutment with each other and fusion bonded to each other. Thus, the following advantages are obtained. That is, the abutting operation in the abutment portions is simple and easy, and the dividing surfaces are along the direction of a main magnetic flux (diametral direction), so reduction in the output power of the alternator can be suppressed.
In the case of the above-mentioned stator core, the element iron core of the hexahedral shape is made into a cylindrical shape and its opposite end faces are welded together so as to be joined to each other, but a considerable force is required for bending the element iron core, and a spring back after bending of the element iron core is also strong, so a gap is liable to occur between the joining faces of the element iron core after the cylindrical formation thereof. If such a gap occurs and grows large, magnetic flux across the gap might be adversely influenced so that it might be disturbed to deteriorate magnetic noise, and in addition to this, a tensile force might be applied to coil ends of the stator winding fitted in the slots thereby to damage them.
In addition, it is known that the stator core fluctuates minutely in a diametral or radial direction thereby to worsen electromagnetic noise due to the resonance at the natural frequency of the stator with a magnetic exciting force generated when magnetic flux or electric current flows through the stator, but the stator core itself has most portions of its outer periphery exposed to the outside, and hence is not constructed so as to suppress the diametral resonance.
As a countermeasure for such a problem, it is considered that the opposite end faces of the element iron core are strongly joined or bonded to each other by fusion bonding the entire surfaces (region of A-B-C-D) of the abutted portions in a tooth, as shown in FIG. 8.
Here, note that in FIG. 8, an axial length Lc of the stator core is longer than a diametral or radial length d thereof. This is because the diametral length d of the stator core is made shorter and the axial length Lc of the stator core is made longer to simply increase a region thereof through which magnetic flux passes, so as to meet a demand for increased output power and increased efficiency of an vehicular alternator in recent years.
In general, however, oxidation products are generated on the surfaces of the abutment portions fusion bonded to each other, and the oxides (e.g., Fe2O3 in case of rolled steel sheet) thus generated have a large resistance, so if such oxidation products extends over the entire width or length C-D of the diametrally inner peripheral surface of the tooth through which a main magnetic flux of a magnetic circuit passes, the performance is lowered undoubtedly, resulting in a major hurdle to the commercialization of the vehicular alternator.
Moreover, when the entire surfaces of the region A-B-C-D are fusion bonded to each other, eddy currents, which are suppressed by the lamination of a multitude of thin plates in the form of rolled steel plates, will flow into the region A-B-C-D, thus lowering the efficiency.
Further, to perform welding so as to fusion bond the entire surfaces of the region A-B-C-D to each other necessarily increases the time for welding operation and at the same time enlarges the size of welding equipment as well.